1. Field of the Invention
This invention relates to an electric inspection unit using an anisotropically electroconductive sheet and a process for producing the anisotropically electroconductive sheet.
2. Discussion of the Background
Anisotropically electroconductive sheets are useful as materials for achieving electrical connection in various applications because of their structural characteristics.
However, in the inspection of a base board for a printed wiring board or the like, pin probes have heretofore been used as materials for achieving electric connection.
The pin probes are, however, difficult to miniaturize because of their structural restriction. Accordingly, they are inappropriate for electric inspection of a base board which now tends to be made as fine as possible. Moreover, when the electrode in a portion to be inspected is a surface electrode, a point contact is formed, and therefore, a bad contact tends to be caused. In particular, pin probes are quite inadequate for electric inspection of a base board for surface-mounting technology (referred to hereinafter as the SMT base board) which is now being widely used.
On the other hand, anisotropically electroconductive sheets have no such structural limitations as in the case of pin probes in the electric inspection of a base board, and can be produced in such a form as to be applicable to a conduction-insulation test of printed wiring having high mounting density, a representative of which is the SMT base board.
Anisotropically electroconductive sheets which have heretofore been known are those having the following structures:
(1) Structure in which plural insulating rubber sheets are alternately laminated to plural electroconductive rubber sheets filled with electroconductive carbon particles (see Japanese Patent Application Kokoku No. 56-48,951).
(2) Structure in which metal particles are uniformly dispersed in an elastomer sheet (see Japanese Patent Application Kokai No. 51-93,393).
(3) Structure in which electroconductive magnetic particles are non-uniformly distributed in an elastomer sheet (see Japanese Patent Application Kokai Nos. 53-147,772 and 54-146,873).
However, anisotropically electroconductive sheets having the above structures (1) to (3) are all flat, and hence, it is necessary to apply a large pressure thereto for forming effective electric paths. In addition, in the case of miniaturized printed wiring boards and the like, there are many points to be inspected, and the total necessary pressure becomes considerably large and there is such a problem that the pressing mechanism of inspecting unit must be unusually strong.
Under such circumstances, a proposal has been made of an anisotropically electroconductive sheet structure so that a difference in height is formed between the surface of an electroconductive portion and the surface of an insulating portion (see Japanese Patent Application Kokai No. 61-250,906).
When an anisotropically electroconductive sheet having a structure in which such a difference in height is formed is used, the total necessary pressure can be reduced.
However, the above anisotropically electroconductive sheet is prepared by a press-molding in the prior art; therefore, a press mold to be used in the press-molding must be prepared in conformity with the shape and size of an anisotropically electroconductive sheet each time. Also, the mold must have a structure which can withstand press-molding. Moreover, when the above anisotropically electroconductive sheet is formed by press-molding, it is difficult to release from the mold because the sheet has a structure in which such a difference in height is formed.
On the other hand, conventional electric inspection unit in which an anisotropically electroconductive sheet is used adopts a so-called intermediary pin system as shown in FIG. 7 in the accompanying drawings. In FIG. 7, 91 refers to an inspection head provided with many inspection electrodes 96, 92 to an intermediary pin system, 93 to an off-glid adaptor, 94 to a printed wiring board and 95 to a back side adaptor.
The intermediary pin system 92 consists of an anisotropically electroconductive sheet 92A, an intermediary pin 92B and another anisotropically electroconductive sheet 92C and has a function of absorbing errors in dimension in the direction vertical to the surface to be inspected of the printed wiring board 94 and a function of preventing electric current from spreading in the transverse direction in the interiors of the anisotropically electroconductive sheets 92A and 92C by the intermediary pin 92B per se to keep the point-to-point correspondence between the inspection electrodes 96 and the portions to be inspected.
However, an electric inspection unit in which such an intermediary pin system is adopted is complicated in structure, has a difficulty in production, and in addition, is so heavy in weight that its handling is not easy. Moreover, the unit has many points at which electric current flows upon contact, and hence, is inferior in reliability.